The thought came into my head "I wonder what order all the Sonic games fall next to one another in a full consistent timeline". Let's cover the obvious ones... (Skip to the end for the Part 1/3's timeline).

Sonic the Hedgehog
Sonic the Hedgehog 2
Sonic 3 & Knuckles

Now let's try to place the game Sonic CD some where... Now it can't go in between Sonic 2 and 3 because the latter is a direct follow up to the former so it ether happens before Sonic 2 or after Sonic 3... There's no overwhelming evidence for ether except maybe the easter egg with Tails saying "See you next game!" but since that is in fact an easter egg it probably shouldn't count. So our new time line looks like this...

Sonic the Hedgehog
Sonic CD?
Sonic the Hedgehog 2
Sonic 3 & Knuckles
Sonic CD?

Now we should look at the Sonic the Hedgehog 4 Saga and since it is stated that Episode I takes place after Sonic 3 & Knuckles and the future episodes take place right after Episode I the timeline should look like this...

Sonic the Hedgehog
Sonic CD?
Sonic the Hedgehog 2
Sonic 3 & Knuckles
Sonic CD?
Sonic the Hedgehog 4: Episode I
Sonic the Hedgehog 4: Episode Metal
Sonic the Hedgehog 4: Episode II

In Episode II Sonic CD's Little Planet returns which comes around in a unspecified amount of time. I doubt it comes back so often that it would only take the time between Sonic CD and Episode I and the 2 days the former takes place (We see the passing of time in Ep. I). So the more likely time line is this (But Sonic CD taking place after 3 is still a possibility just slightly less of one):

Sonic the Hedgehog
Sonic CD
Sonic the Hedgehog 2
Sonic 3 & Knuckles
Sonic the Hedgehog 4: Episode I
Sonic the Hedgehog 4: Episode Metal
Sonic the Hedgehog 4: Episode II

Now we've placed all the main games from the classic era. At this point we should focus on the side games. Let's start with the Game Gear games! I think it's reasonable to say that the games that have Tails in them and less than 7 chaos emeralds is non-canon as Sonic 2 (Genesis) is his confirmed first appearance canonically in the series. Which only leaves Sonic the Hedgehog (GameGear) and that's just a different version of the same adventure in it's Genesis counterpart. So it can be changed out with the original but i'm going to leave it off the timeline for convenience.

For Sonic Spinball both versions contain to many chaos emeralds and have characters from the cartoon and thats evidence enough that Sonic Spinball is not canon.

Knuckles' Chaotix for the 32X could not have happened because Metal Sonic disappeared with Little Planet and is the final boss of this game. Now Eggman could have built a new one
but for the sake of simplicity we'll just say it's not canon but keep in mind it is possible and if it was it would take place after Sonic 3 & Knuckles but before Sonic 4 due to the Japanese manual saying " a mysterious island rose from the sea shortly after the events of Sonic & Knuckles." and the designs are still the original.

Now for the last "Classic" game, Sonic 3D Blast! Now this game has both Tails and Knuckles and their original designs making it take place after S3&K but before Sonic 4 Ep. I. If Knuckles' Chaotix was in the equation it would most likely take place before 3D Blast due to the previous quote from the Japanese manual.

Sonic the Hedgehog (Genesis or GG)
Sonic CD
Sonic the Hedgehog 2
Sonic 3 & Knuckles
Sonic 3D Blast
Sonic the Hedgehog 4: Episode I
Sonic the Hedgehog 4: Episode Metal
Sonic the Hedeghog 4: Episode II

The next game I want to cover is Sonic Pocket Adventure which shows Eggman changing from his original design to his updated one in the middle putting this game after 3D Blast but before Sonic 4 Ep. I. But to do this we have to assume that the new designs started at this point though this wouldn't really change any of the progress we've made.

Sonic the Hedgehog (Genesis or GG)
Sonic CD
Sonic the Hedgehog 2
Sonic 3 & Knuckles
Sonic 3D Blast
Sonic Pocket Adventure
Sonic the Hedgehog 4: Episode I
Sonic the Hedgehog 4: Episode Metal
Sonic the Hedeghog 4: Episode II

Before we move on to the modern era I should mention Sonic Drift can take place after Sonic 2 because of the characters and this also confirms CD's place in the timeline. But for Sonic Drift 2 It features a character from a non-canon game (Sonic Triple Trouble which is non-canon because it has 6 chaos emeralds but features Tails and Knuckles) which is Fang.
Speaking of racing games Sonic R for the Saturn includes Metal Sonic making it possible but not likely. P.S. Sonic the Fighters has one to many chaos emeralds. Making this today's final timeline:

Sonic the Hedgehog (Genesis or GG)
Sonic CD
Sonic the Hedgehog 2
Sonic Drift
Sonic 3 & Knuckles
Sonic 3D Blast
Sonic Pocket Adventure
Sonic the Hedgehog 4: Episode I
Sonic the Hedgehog 4: Episode Metal
Sonic the Hedeghog 4: Episode II

Make sure to watch out for Part 2/3- Dreamcast Era. Make sure to comment if you find anything wrong with this one and i'll be sure to change it. See ya soon!

Oh yeah anyone opposing my Metal Sonic theory i'm definitely open to that but please explain where the second Metal Sonic went in Sonic 4 Ep. II and why Eggman would get rid of it for the 1st, broken one. The goal here is to make a consistent Sonic the Hedgehog timeline and your help would be greatly appreciated.

Part 2/3- link